Laurel, MT, USA
N9104S
Thurmond Moni
The pilot of the homebuilt aircraft and installer of the wooden propeller assembly reported that during a performance test/rate of climb test with the new propeller, he smelled a wood burning smell followed by a "real bad vibration." The pilot initiated a forced landing to an open field. During the landing roll on the soft terrain, the landing gear sheared off. After the accident, the pilot noted that one of the blades of the wooden propeller was missing. The remaining blades and two hub plates were sent to the NTSB Materials Laboratory for examination. The metallurgist reported that the fretting observed on the hub plates suggested that the hub mounting bolts were not as tight as required allowing the hub to oscillate on its mounting flange. The motion in the pitch block of both blades produced the heat that charred the wood and softened the pitch block material allowing it to be displaced and imprinted with the pattern of the wood plies. The compression failure and the frictional features on the hub portion in the pitch block indicated a fatigue-like failure in wood which propagated until the remaining wood could not withstand the stresses of operation and subsequently failed in a tensile mode.
On July 12, 2002, about 1020 mountain daylight time, a homebuilt Moni motorized glider, N9104S, registered to and operated by the pilot as a 14 CFR Part 91 personal flight, experienced an in-flight separation of the propeller blade. The pilot initiated a forced landing to a field near Laurel, Montana. During the landing roll, the landing gear sheared off. Visual meteorological conditions prevailed at the time and no flight plan was filed. The glider was substantially damaged and the private pilot, the sole occupant, was not injured. The flight departed from the airport at Laurel about 20 minutes prior to the accident. The pilot reported that the purpose of the flight was for a performance test/rate of climb test with a new propeller. The pilot stated that he smelled a wood burning smell. He checked all the gauges which were okay. A few seconds later, the pilot heard a "buzzing" sound followed by a vibration. The pilot decreased engine power, then increased the power and the sound and vibration followed the engine changes also decreasing and increasing. About 30 seconds later, the pilot felt a "real bad vibration" and he shut the engine off. The pilot initiated a forced landing to a field. During the landing roll on the soft terrain, the landing gear sheared off. After the accident, the pilot noted that one of the blades of the wooden propeller was missing. Maintenance records indicated that the Precision Propellers, Inc., ground adjustable, model 34 inch LE XT, serial number 4828, wood propeller was manufactured in September 2000. The propeller and spinner had been installed by the pilot on June 2, 2002. The pilot reported that the pitch had been set to 11 1/2 inches. Approximately 5.3 hours had been accumulated since the installation of the propeller. On June 29, 2002, and 4.4 hours since the installation of the propeller, the maintenance logbook indicated that during a post-flight inspection the pilot found a "sheared prop bolt at head, bolt #4. The bolt was replaced with a new AN5-31A bolt, and the flywheel was torqued to 37 ft-lbs. The propeller bolts were torqued to 150 in-lbs. The propeller hub, which included the two hub plates, the pitch blocks, tubing spacers and associated hardware were provided for inspection. It was noted that one end of the hub contained within the pitch block halves what remained of the butt end of one of the wooden propeller blades. Approximately 1/3 of the total material of the wood was darkened. A small section of the matching material that separated was also provided and that surface was also darkened. Only fragments of wood material remained within the pitch block halves for the other propeller. Within the pitch block halves for this side, evidence of what appeared to be darkened wood material was visible. Fragments of light colored wood material was also present. The propeller hub and wood fragments were sent to the NTSB Materials Lab for inspection. The Mechanical Engineer reported that a portion of blade root was extracted from pitch block "A". The engineer reported that the examination revealed that approximately 80% of the fracture face consisted of jagged features consistent with a tensile fracture. The remaining 20% of the fracture face consisted of a relatively flat and discolored area. The discoloration was black adjacent to the outside surface and became lighter as it progressed inwards. Pitch block "B" revealed two pieces of blade root material, deformation of the block material and parallel lines impressed on the inner surface. The pitch blocks contained significant deformation in the radially outboard direction. Removal of the two wood pieces revealed significant deformation in the pitch block under these pieces. Wide, thin slivers of pitch block material were observed at one location in each of the two locking vees for the blade root. The slivers were adjacent to the block parting line, approximately 180-degrees apart and oriented inboard. The propeller pieces were then shipped to the Department of Agriculture, Forest Products Laboratory for additional examination. The examination confirmed that the jagged fracture faces were consistent with a tension failure and that the flat areas were consistent with a compression failure. Examination of the discolored areas revealed that the almost crystalline, blackened appearance was consistent with charring. They also determined that the characteristics of the cellular structure of the propeller blades were consistent with a hard maple, either Acer saccharum or Acer nigrum.
Undertorqued mounting bolts which resulted in fatigue to the wooden propeller blade and subsequent in flight separation of one of the blades while in cruise flight. An improper installation of the propeller assembly and soft terrain encountered during the landing roll were factors.
Source: NTSB Aviation Accident Database
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